LFOModifier: gate output by MIDI activity in note-trigger mode

[lucaromagnoli]

Summary

• Adds setGateOnTriggerSource(bool) to LFOModifier. When set and syncType == note, applyToBuffer drives the existing gated_ flag from the MIDI stream itself: NoteOn ungates and increments a held-note count, NoteOff (vel-0-aware) decrements it, AllNotesOff / AllSoundOff hard-resets. Gate re-asserts once the count returns to zero.
• Lets MIDI-trigger LFOs behave like envelopes — output sits at 0 between notes — without changing the free-run sync mode for callers that don't opt in.

Why

Previously a note-mode LFO only resynced phase on note-on; its output free-ran between MIDI notes, which forced consumers to maintain a parallel "is the LFO running" simulation just to make UI animations and audio agree. With this opt-in flag, the audio engine itself owns the running state.

Behavior

• Disabled by default → no change for existing TE callers.
• When enabled + syncType == note: gate follows MIDI activity, output sits at 0 between notes.
• When enabled but syncType != note: flag is ignored (still no gate).
• Disabling the flag clears any held gate so output resumes immediately.

Test plan

• Enable on a MIDI-trigger LFO → output is 0 with no notes held, full waveform while a note is held, returns to 0 on note-off.
• Disable the flag at runtime → output resumes mid-note correctly.
• Default (flag off) behavior matches upstream (LFO free-runs between notes).